This invention relates to video signal timing in a multi-format environment.
Until quite recently, the rules of the United States Federal Communications Commission (FCC) required that video material should be broadcast using the NTSC signal format. The NTSC video format is an interlaced format, in which each frame is composed of two fields. The frame rate of the NTSC video format is 30/M frames/sec where M is a data rate divisor and is equal to 1.001.
In a television broadcast facility, several video signal sources, each generating a signal in NTSC format, are connected to respective inputs of a router, which supplies the video signals to selected video signal destinations. The various signal sources are controlled by a common timing signal so that they are in phase.
The router is composed of multiple input buses, connected to the sources respectively, multiple output buses, connected to the destinations respectively, and an array of crosspoint switches that are operable selectively for connecting any one input bus to any selected set of one or more output buses. The state of the ij th crosspoint switch is ON (it connects output bus j to input bus i) or OFF (it isolates output bus j from input bus i). The router includes a memory having an addressable location associated with each crosspoint switch. During each active interval of the video signal, the memory is addressed to store in each location the desired state of the corresponding crosspoint during the next active interval. The desired state may be the same as the current state or it may be the opposite state. During the vertical blanking interval, the contents of the memory are used to update the entire array of crosspoint switches.
In order to minimize degradation of the output signals of the router, the update is made consistently on the same line of the video signal frame. This line is referred to as the switch line, and the time at which the crosspoints are updated is referred to as the switch point. Nominally, the switch point is halfway through the switch line, but SMPTE RP-168 defines a window of +/-5 .mu.s.
Since the NTSC video format is an interlaced format, there are two vertical blanking intervals, and hence two switch lines, per frame. In each field, the switch line is the first line of the vertical blanking interval after the equalizing pulses. The switch line is identified by continuously monitoring the video signal to find vertical sync, and counting lines until the first line after the equalizing pulses.
Current rules of the FCC allow broadcast of video material in a high definition (HD) format, but require that a television broadcaster who broadcasts video material in an HD format should broadcast the same material in standard definition (SD) NTSC format. One way for a broadcaster to meet this requirement would be to generate the video material for broadcast in the HD domain and, concurrently with broadcast of the HD material, convert the HD material to SD and broadcast the SD material. An alternative, which may be attractive for some purposes, would be to receive the source material in both formats, process the SD material in an SD path to generate the SD broadcast stream and concurrently process the HD material in a separate, parallel HD path to generate the HD broadcast stream, and broadcast the SD material and the HD material simultaneously.
Although the SD video signals that are currently broadcast are analog formats, the HD signals are in digital format and almost all of the processing of SD and HD video signals is conducted in the digital domain.
At least thirteen digital HD signal formats, designated by SMPTE 292M Format ID's A-M, have been defined for HD video signals. Several of the format parameters for these digital high definition video signals are listed in Table I.
TABLE I Reference SMPTE 295 Standard 260 M M 274 M 296 M Format ID A B C D E F G H I J K L M Lines per frame 1125 1125 1250 1125 1125 1125 1125 1125 1125 1125 1125 750 750 Total active lines 1035 1035 1080 1080 1080 1080 1080 1080 1080 1080 1080 720 720 Frame rate (Hz) 30 30/M 25 30 30/M 25 30 30/M 25 24 24/M 60 60/M Fields per frame 2 2 2 2 2 2 1 1 1 1 1 1 1
The U.S. broadcast facilities that simultaneously broadcast HD and SD streams have each selected either format E or format M for digital HD material. Format E is a 30/M frame/sec, 1125 lines/frame interlaced format and format M is a 60/M frame/sec, 750 lines/frame progressive scan format.
For some of the digital HD signal formats, there are equivalent analog signal formats in which the blanking interval includes equalizing pulses. Depending on the format, the equalizing pulses occur in the first five or six lines of the vertical blanking interval. With respect to a digital signal format having a corresponding analog format, it has been proposed that the switch line should be the first line after the vertical blanking interval. It has also been proposed that the switch line for a digital signal format for which there is no corresponding analog format should be the same line as the switch line for the closest digital format for which there is a corresponding analog format.
In video post production, a router is used to assemble source material to produce a video stream which is supplied to a broadcast facility to include in a broadcast stream. For example, in post production several brief scenes might be assembled to form a commercial message, which the broadcaster combines with other commercial messages and program material to generate the broadcast stream. In post production, the segments of the program source material may be in different formats. It may be necessary to combine a segment of a first format with a segment of a second format and produce a program in a third format. Much of the material used in post production is based on film, which is 24 frames/sec.
SMPTE 259 specifies the structure of a serial digital interface (SDI) data stream derived from a 10-bit 4:2:2 component digital signal or a 4f.sub.sc NTSC composite digital signal. The SDI data stream was specified in order to facilitate interconnection of equipment from different manufacturers. Each line interval of the SDI data stream has four areas: EAV (end of active video) for the previous line, blanking level or horizontal ancillary data, SAV (start of active video) and digital active line, which contains active picture or vertical ancillary data depending on the location of the line in the frame.
Similarly, SMPTE 292M specifies the structure of a serial digital interface data stream derived from 10-bit source data words for HD digital signals. Digital video data in accordance with any of the SMPTE HD formats defined in Table I can be distributed in a serial data stream in accordance with SMPTE 292M.
The serial digital interface data stream specified in SMPTE 292 does not include a word that specifies the source format, e.g. SMPTE format E or SMPTE format M, and accordingly it is not possible to determine the source format without disassembling the data stream and observing patterns in the source data. Consequently, it is not possible to determine whether there is a corresponding analog format and, if so, whether the equalizing pulses of the corresponding analog format occupy the first five lines or the first six lines of the vertical blanking interval.